1. Technical Field of the Invention
The present invention relates generally to electromagnetic switches, and more particularly to electromagnetic switches for engine starters of motor vehicles.
2. Description of the Related Art
There is known a conventional electromagnetic switch for controlling supply of electric power to an electric motor in an engine starter of a motor vehicle (see, for example, Japanese Utility Model Application Publication No. S60-102469).
Specifically, the conventional electromagnetic switch includes an electromagnetic coil, a movable core, a pair of fixed contacts, and a movable contact. The fixed and movable contacts together make up main contacts of a motor circuit for supplying electric power to the motor.
In operation, when the electromagnetic coil is energized, the movable core is attracted by a magnetic attraction, which is created by the energization of the electromagnetic switch, to move the movable contact, thereby causing the movable contact to bridge or electrically connect the fixed contacts. As a result, the main contacts of the motor circuit are closed so that electric power is supplied from a battery to the motor.
Further, when the energization of the electromagnetic coil is interrupted, the magnetic attraction disappears, causing the movable core and the movable contact to return to their initial rest positions. Consequently, the fixed contacts are electrically disconnected. As a result, the main contacts of the motor circuit are opened so that the supply of electric power to the motor is interrupted.
Moreover, it is also known that in such a conventional electromagnetic switch as described above, when the energization of the electromagnetic coil (or the supply of electric power to the electromagnetic coil) is interrupted by an energization interrupting means, some damage may be made to the energization interrupting means due to the inductance of the electromagnetic coil.
In particular, in the case of the energization interrupting means being made up of an electric relay that includes a pair of fixed contacts and a movable contact, an electric arc may be generated between the fixed contacts and the movable contact when the electric relay interrupts the energization of the electromagnetic coil, thereby causing wear of the fixed and movable contacts.
Furthermore, in recent years, idling stop systems (or stop-start systems) have come to be used in motor vehicles for improving the fuel economy. For an engine starter used in a motor vehicle that is equipped with an idling stop system, the number of times the engine starter starts the engine of the vehicle is dramatically increased in comparison with an engine starter used in a motor vehicle without an idling stop system. Therefore, for the electromagnetic switch used in such as an engine starter, it is particularly important to ensure high durability of the energization interrupting means.
To solve the above problem, Japanese Utility Model Application Publication No. S60-10246 discloses a technique according to which: a diode is electrically connected in parallel with the electromagnetic coil of the electromagnetic switch, with its anode arranged to be lower in electric potential than its cathode. Consequently, when the energization of the electromagnetic coil is interrupted by the energization interrupting means, at least part of the electric current, which is caused by the inductance of the electromagnetic coil, can be made to circulate within a closed circuit formed by the electromagnetic coil and the diode, thereby minimizing the amount of the electric current flowing to the energization interrupting means.
Moreover, in a motor vehicle, the engine starter is generally directly fixed to the engine of the vehicle. Therefore, the electromagnetic switch used in the engine starter is generally subject to high vibration. Further, when the diode is disposed within the electromagnetic switch, the diode may receive a mechanical shock which is caused by the movement of the movable core in the axial direction of the electromagnetic switch. Therefore, it is desired to reliably protect the diode from the mechanical shock.